This invention relates to the use of memories in electronic postage meters and, more particularly, to the use of multiple forms of memory including the instantaneous transfer from volatile to non-volatile memory for protection of information during power failure.
Electronic postage meters may incorporate a computer, typically a microcomputer or microprocessor, to provide a variety of functions. These functions include, among others, a tally of postage costs and remaining postage already paid for; the number of mail pieces which have been stamped; maintenance of the proper date; and similar functions useful in the processing of mail. Such computers are utilized with memories which store original data, such as the amount of postage originally paid for, and new data, such as the current amount of postage utilized. The program by which the computer is operated is also stored.
In the use of memories, a distinction is made between live or volatile memories, wherein stored data can be continually changed, and fixed or non-volatile memories wherein data can be stored permanently, independently of the presence of electric power. The live memory is employed continuously in the computer operations, particularly in arithmetic operations. The fixed memory may be used for storage of results of the computations as well as for storage of the program.
A problem arises in that with present day techniques in the construction of memories, the volatile memory can retain its stored data only by the use of continuous electric power inputted to the memory. In the event of power failure, the data is lost. The random-access memory (RAM) is one such memory. In contrast, the fixed memory, such as the read-only memory (ROM) or electrically eraseable and programable ROM (EEPROM), retains its stored data even without electric power. However, only the EEPROM is suitable for storage of alterable data. Unfortunately, EEPROMs at the present time are slow and have a limited life. This makes it impractical to use an EEPROM instead of RAM for the storage of variable data except when it is necessary to preserve such data after power is turned off.